Top left is the original M-250A2 (originally called the M-150) with the squared nose and top. 1987 was the last year those were made. I have one that was made in 1987. The top right and bottom right are both the second generation M-250A2, made from 1987 through the late 90s. Those had the same squared nose (refractor doors are interchangeable with the 1st generation) but the top was rounded off. The bottom left is the third and current generation, which has a round nose and top. The refractor doors are not interchangeable with older models, though the Powr/Doors are and remain unchanged other than the fact that the original one had the GE logo upside down and the newer one has the logo facing the same way as the NEMA tag.

The top left one was made between ~1984 and 1987. The top right was most likely made in 1989-1990 (NEMA tag font). The bottom left was made sometime before 2005 and the bottom right was made sometime around the mid-90s.